1. Field of the Invention
The present invention relates to an image forming apparatus, such as copying machines, and a control method therefor.
2. Description of the Related Art
Conventionally, when both sides of multiple documents are copied by an analog image forming apparatus that does not have a function of storing read original image information, the following approach is adopted. That is to say, image formation on the back side of a sheet is performed in such a way that a sheet with an image formed on one side by an image forming unit is not discharged outside the apparatus, but temporarily stacked on an intermediate tray, and upon reading an original image associated with the back side of the sheet, the sheet is re-fed to the image forming unit.
In contrast to this, an image forming apparatus having a function of storing the read original image can achieve a through-pass double-sided copying function, which does not have to stack the sheet with an image formed on one side, in double-sided copying (see, e.g., Japanese Laid-Open Patent Publication (Kokai) No. 2002-145535).
Next, the operation method of double-sided copying in the through-pass double-sided copying type image forming apparatus will be described with reference to FIG. 10.
FIG. 10 is a view schematically showing a configuration for illustrating the operation method of double-sided copying in the through-pass double-sided copying type image forming apparatus.
First, for example, an automatic document feeder feeds sheets, and all pages of a predetermined number of documents are read by a scanner, and stored in a storage unit (not shown). For example, when the document is a double-sided document, both sides are read, thereafter, images of a predetermined number of sheets, for example, three sheets of odd-numbered pages (the first page, the third page, and the fifth page) of a document are sequentially read from the storage unit. Then, after copying is sequentially performed on the front faces (first side) of the sheets, which are sequentially fed from a sheet feeding unit 601 in an image forming unit 602, three front face copied sheets P-1, P-2 and P-3 are reversed by a switchback unit 603. Subsequently, the sheets are sequentially transported in a double-sided path 604 to the image forming unit 602, and stopped in a first-to-last position relationship (in a non-stacked state).
When a first image of an even-numbered page is formed on the back face (second side) of the sheet, the first (head) sheet P-1 among the three sheets stopped within the double-sided path 604 is re-fed to the image forming unit 602 (hereinafter, this sheet re-feeding is referred to as the “double-sided sheet re-feeding”). Then, a toner image corresponding to the image of the second page read from the storage unit is formed on the back side of the first sheet P-1. At that time, the remaining two sheets P-2 and P-3 in the double-sided path 604 sequentially go ahead in preparation for the double-sided sheet re-feeding.
Next, the fourth sheet is newly fed from the sheet feeding unit 601 to have an original image of the seventh page formed on the front face, and is transported to the rearmost of the double-sided path 604 and stopped. Then, the second sheet P-2 stopped at the front end in the double-sided path 604 is fed to the image forming unit 602 in a double-sided sheet re-feeding fashion, and the original of the fourth page is copied on the back side thereof. Subsequently, a fifth sheet is newly fed to the image forming unit 602 from the sheet feeding unit 601, and the original of the ninth page is copied on the front face thereof.
In this manner, when the through-pass double-sided copying function is implemented, the double-sided path 604 is filled up with the single-sided copied sheets, and then feeding of a new sheet from the sheet feeding unit 601 and the double-sided sheet re-feeding are alternately performed.
Meanwhile, in recent years, since color printers have become widespread and their image quality has improved, the size of image data has been increased. One way of dealing with increased image size is compressing the image data saved in a hard disk drive (hereinafter referred to as the “HDD”). Before image formation is performed, the image data is read from the HDD and decompressed. Further, printers have emerged which need much time for image data processing because functionality of the printers is expanded and an original image is subjected to various types of image processing; thus further time is needed for preparation of image data before image formation is performed.
Above all, some image forming apparatuses using an intermediate transfer body, for example, generate the timing of sheet feeding from a sheet feeding unit based on the timing of the start of image formation because the time required to form a toner image is longer than the time required to feed and transport sheets. In such apparatuses, since image formation must be started before the timing of the start of double-sided sheet re-feeding, the preparation time of image data must be within a predetermined time period.
Under such circumstances, if a delay in the preparation time of data of an image to be formed is caused due to delay in reading the data from the HDD or the like, the double-sided sheet re-feeding of the sheet P-1 stopped at the front end in the double-sided path 604 is delayed. As a result, the number of sheets that are circulating through the apparatus (the number of sheet circulation pages) sometimes exceeds the number of positions where single-sided copied sheets are stopped within the double-sided path 604 (the number of sheet stop positions). In such a case, there is a problem that a malfunction such as jamming occurs due to a collision of a later sheet with an earlier sheet.
For example, a case is assumed where, in a configuration in which there are three sheet stop positions as shown in FIG. 10, the number of the sheet circulation pages is seven. That is to say, a case is assumed where the sheet feeding unit is controlled to initially continuously feed four sheets (the first page, the third page, the fifth page, and the seventh page) for first side copying (see FIG. 7B described later). After a minimum time (see ST in FIG. 5 described later) required for a sheet to circulate through the apparatus for first side copying and to be re-fed for second side copying has elapsed, the sheet of the seventh page is fed from the sheet feeding unit. Then, the head sheet (corresponding to P-1 in FIG. 10) stopped at a stop position in the double-sided path 604 is re-fed as a second page.
At that time, if the double-sided sheet feeding of the sheet of the head second page is delayed, a full state is reached in which the sheets are waiting at all the three stop positions within the double-sided path 604. As a result, since there is no stop position for the first side-copied sheet of the seventh page, the sheet of the seventh page collides with a sheet waiting at the rearmost stop position (corresponding to P-3 in FIG. 10).
Conventionally, in order to avoid such a malfunction, the number of the sheet circulation pages X has been determined so as not to exceed the number of the sheet stop positions Y (X=2Y−1). That is to say, in the configuration example in FIG. 10 in which there are three sheet stop positions, the number of the sheet circulation pages has been controlled to be five, that is, the sheet feeding unit has been controlled to initially continuously feed three sheets (the first page, the third page, and the fifth page) for first side image formation (see FIG. 7B described later).
However, in such control, the number of the sheet circulation pages cannot be increased, thus productivity is adversely affected. Accordingly, in order to satisfy productivity, a method of enhancing the speed of sheet transport and a method of increasing sheet stop positions have been known, and, for example, there is a method of controlling the speed of sheet transport by sheet size (e.g., Japanese Laid-Open Patent Publication (Kokai) No. 2005-280897).
As described above, the method of enhancing the speed of sheet transport in order to satisfy productivity needs an expensive drive motor or the like, resulting in increase in cost. Further, for the method of increasing sheet stop positions, there is a problem that the apparatus is upsized.